1. Field of the Invention
The present invention generally relates to an apparatus and method for obtaining three-dimensional images and in particular to a synthetic array radar (SAR) capable of obtaining three-dimensional images using aperture elements distributed along a curvilinear path in space without having to position an antenna at all element positions of a filled two-dimensional spatial array.
2. Description of the Prior Art
The use of three-dimensional supplemental information for enhancement of the interpretation of two-dimensional imagery is well recognized in the field of aerial photography. Two broad advantages of a three-dimensional image over a two-dimensional image are apparent. For example, the separability of unwanted clutter from areas where targets are likely to exist may bring about a higher signal to clutter radio. Thus, returns from a tree canopy or those from the ocean surface may be removed or attenuated for better interpretation of the target volume. Secondly, a three-dimensional target signature of a certain resolution cell size contains more information than its two-dimensional counterpart. This promises to be a more effective discriminant for use in a search or identification procedure.
Two-dimensional imaging by aperture synthesis is widely employed using a coherent radar whose transmitting and/or receiving antenna(s) is/are in motion relative to the object whose image is desired. Thus, synthetic aperture radar (SAR) maps of terrain are provided by flying radar platforms along linear paths and recording and processing wideband radar data from time periodic transmitter-receiver element positions. The second "aperture" dimension is provided by varying the radar frequency over a wide bandwidth.
A two-dimensional aperture in this context is an array of points in spatial frequency whose direction and distance from the origin of spatial frequency coordinates are determined by the radar line-of-sight and wavelength respectively ("Quantitative Coherent Imaging: Theory, Methods and Some Applications", by J. M. Blackledge, Academic Press, 1989). Similarly, inverse synthetic aperture radar (ISAR) achieves two-dimensional imaging using wide bandwidth and target rotation about a fixed axis to provide the requisite relative motion as discussed in "Developments in Radar Imaging" by Dale A. Ausherman, et al., IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-20, No. 4, July 1984. The apertures synthesized in these cases are typically "filled," i.e., radar frequencies are uniformly spaced throughout the bandwidth and antenna element positions are uniformly spaced along the radar platform trajectory. The element spacing is set to conform to the Nyquist sampling criterion and element strengths are weighted to control sidelobes of the imaging system point spread function (PSF). Hence, conventional processing employs a two-dimensional Fourier transform of the data array to obtain an image.
The above techniques may in principle be applied to accomplish three-dimensional radar imaging using antenna positioning in a two-dimensional spatial array along with wide radar bandwidth to obtain three "aperture dimensions" as shown in FIG. 1A. The obviously perceived difficulty lies in physically positioning the antenna at all element positions of a filled two-dimensional spatial array. It is impractical, if not impossible, to fly a radar platform so as to sequentially occupy all the element positions of a regular lattice or to have a target object move so as to achieve a similar relative positioning. Also, aperture element positions (and hence PSF's) are not precisely known in most SAR and ISAR imaging applications. As a result, it has been necessary to provide motion compensation (in the form of phase corrections of the radar data) or autofocus (automatic adjustment of element phases to achieve focus) in order to make these systems perform acceptably. (G. A. Bendor and T. W. Gedra, "Single-Pass Fine-Resolution SAR Autofocus," NAECON83, Vol. 1, 1983, pp. 482-488; and Rongquing Xu, Zhidao Cao, and Yongtan Liu, "A New Method of Motion Compensation for ISAR," IEEE International Radar Conf. Proc. 1990, pp. 234-237, the contents of which are incorporated herein by reference).
In spite of all this, it remains desirable to have a three-dimensional processing system which has data compatible with that taken for what are known as two-dimensional spotlight and strip-map mode SAR operation. Moreover, it is desirable to have a three-dimensional system which could be operated in a manner anticipating both modes of operation, i.e. in a two-dimensional mode used for an initial broad area look and in a three-dimensional mode to scrutinize particular regions in that broad area.